U.S. patent number 6,115,403 [Application Number 08/898,710] was granted by the patent office on 2000-09-05 for directly modulated semiconductor laser having reduced chirp.
This patent grant is currently assigned to CIENA Corporation. Invention is credited to Jean-Luc Archambault, Tomas Brenner.
United States Patent |
6,115,403 |
Brenner , et al. |
September 5, 2000 |
Directly modulated semiconductor laser having reduced chirp
Abstract
In accordance with the present invention, an in-line fiber Bragg
grating is coupled to the output of a directly modulated DFB laser.
The grating preferably rejects chirp induced frequencies of light
emitted by the DFB laser. Accordingly, light transmitted through
the grating is spectrally narrowed and has a higher extinction
ratio, thereby decreasing bit error rate probabilities.
Inventors: |
Brenner; Tomas (Severna Park,
MD), Archambault; Jean-Luc (Severna Park, MD) |
Assignee: |
CIENA Corporation (Linthicum,
MD)
|
Family
ID: |
25409929 |
Appl.
No.: |
08/898,710 |
Filed: |
July 22, 1997 |
Current U.S.
Class: |
372/102; 372/20;
372/28; 372/6; 385/37; 398/1 |
Current CPC
Class: |
H01S
3/0057 (20130101); H04B 10/572 (20130101); H04B
10/504 (20130101); H01S 5/0687 (20130101) |
Current International
Class: |
H01S
3/00 (20060101); H04B 10/155 (20060101); H01S
5/0687 (20060101); H01S 5/00 (20060101); H04B
10/152 (20060101); H01S 003/08 (); H04B
010/04 () |
Field of
Search: |
;372/20,25,26,28,32,102,6 ;385/10,37
;359/182,187,188,114,130,161,341 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
PA. Morton et al., "38.5 km Error Free Transmission At 10 Gbit/s .
. . " Electronics Letters, vol. 33 No. 4, pp. 310-311, Feb. 13,
1997. .
K. Sugden et al., "Fabrication of high rejection, low loss, filters
by the concatenation of broadly chirped fibre Bragg gratings", SPIE
vol. 2998, pp. 22-28. .
McCadams et al., CLEO'97, Paper CThW6, pp. 447-448. .
Woodward et al., IEEE Photonics Technology Letters, vol. 5, No. 6,
Jun. 1993, pp. 628-630. .
Lee et al., CLEO'95, Paper Ctu10, pp. 93-94. .
Morton et al., OFC'96 Technical Digest, Paper T4H6, pp.
39-40..
|
Primary Examiner: Sanghavi; Hemang
Attorney, Agent or Firm: Soltz; David L.
Claims
What is claimed is:
1. An optical device, comprising:
a semiconductor laser configured to be coupled to a first end
portion of an optical communication path;
a drive circuit coupled to said semiconductor laser, said drive
circuit supplying an electrical signal to said semiconductor laser
to hereby emit light in a directly modulated fashion;
an optical receiver configured to be coupled to a second end
portion of said optical communication path to thereby receive said
light emitted by said semiconductor laser; and
an in-line fiber Bragg grating provided in said optical
communication path, said in-line fiber Bragg grating being provided
in an in-transmission configuration with an optical output of said
semiconductor laser, said in-line fiber Bragg grating being spaced
a first distance from said first end portion of said optical
communication path and a second distance from said second end
portion of said optical communication path, said first distance
being less than said second distance, said in-line fiber Bragg
grating being configured to substantially reject chirp-induced
frequencies of said light, and said in-line fiber Bragg grating has
a transmissivity characteristic, said transmissivity characteristic
having a first transmissivity over a first range of frequencies and
a second transmissivity, greater than said first transmissivity,
over a second range of frequencies, said first range of frequencies
having a magnitude at least equal to 40 GHz.
2. An optical device in accordance with claim 1, wherein said
in-line fiber Bragg grating has a transmission characteristic as a
function of optical frequency, said transmission characteristic
having a first segment having a first transmissivity, a second
segment having a second transmissivity greater than said first
transmissivity and a third segment between said first and second
segments, said third segment having a slope at least equal to
1dB/GHz.
3. An optical device in accordance with claim 1, further
comprising:
an optical isolator provided in said optical communication path,
said optical isolator being positioned between said first end
portion of said optical communication path and said in-fiber Bragg
grating.
4. An optical device in accordance with claim 1, wherein said light
having a peak optical power at a peak frequency, said in-line fiber
Bragg grating having a loss less than 1 dB at said peak
frequency.
5. An optical device in accordance with claim 1, wherein said
in-line fiber Bragg grating having a loss greater than 10 dB at
said chirp-induced frequencies.
6. An optical device in accordance with claim 1, wherein said
in-line fiber Bragg grating having a loss exceeding 3 dB over a
range of frequencies, said range of frequencies having a magnitude
of 40 GHz.
7. An optical device, comprising:
a semiconductor laser;
a drive circuit coupled to said semiconductor laser, said drive
circuit supplying an electrical signal to said semiconductor laser
to thereby emit light in a directly modulated fashion;
an optical receiver configured to receive a portion of said light
emitted by said semiconductor laser;
an optical coupler coupled to said semiconductor laser; and
a fiber Bragg grating coupled to said optical coupler, said fiber
Bragg grating being configured to substantially reject
chirp-induced frequencies of said light emitted by said
semiconductor laser, said fiber Bragg grating has a transmissivity
characteristic, said transmissivity characteristic having a first
transmissivity over a first range of frequencies and a second
transmissivity, greater than said first transmissivity, over a
second range of frequencies, said first range of frequencies having
a magnitude at least equal to 40 GHz.
8. An optical device in accordance with claim 7, wherein said fiber
Bragg grating has a transmission characteristic as a function of
optical frequency, said transmission characteristic having a first
segment having a first transmissivity, a second segment having a
second transmissivity greater than said first transmissivity and a
third segment between said first and second segments, said third
segment having a slope at least equal to 1 dB/GHz.
9. An optical device in accordance with claim 7, further
comprising:
an optical isolator coupled to said optical coupler and said fiber
Bragg grating.
10. An optical device in accordance with claim 7, wherein said
light having a peak optical power at a peak frequency, said fiber
Bragg grating having a loss less than 1 dB at said peak
frequency.
11. An optical device in accordance with claim 7, wherein said
fiber Bragg grating having a loss greater than 10 dB at said
chirp-induced frequencies.
12. An optical device in accordance with claim 7, wherein said
fiber Bragg grating having a loss exceeding 3 dB over a range of
frequencies, said range of frequencies having a magnitude of 40
GHz.
13. An optical device, comprising:
a semiconductor laser;
a drive circuit coupled to said semiconductor laser, said drive
circuit
supplying an electrical signal to said semiconductor laser to
thereby emit light in a directly modulated fashion;
an optical receiver configured to receive a portion of said light
emitted by said semiconductor laser; and
a fiber Bragg grating coupled to an optical coupler, said fiber
Bragg grating being configured to substantially reject
chirp-induced frequencies of said light emitted by said
semiconductor laser, said fiber Bragg grating having a transmission
characteristic as a function of optical frequency, said
transmission characteristic having a first segment having a first
transmissivity, a second segment having a second transmissivity
greater than said first transmissivity and a third segment between
said first and second segments, said third segment having a slope
with a magnitude at least equal to 1 dB/GHz.
Description
FIELD OF THE INVENTION
This application is related to copending application entitled
"Laser Wavelength Control Under Direct Modulation", incorporated
herein by reference.
The present invention is directed to a system and related method
for narrowing a spectrally broadened output of a directly modulated
laser.
Optical communication systems are a substantial and fast growing
constituent of communication networks. In a typical optical
communication system, information bearing optical signals are
transmitted along an optical fiber. The optical signals are
frequently generated by operating a laser in a continuous-wave (CW)
mode, and modulating the emitted light with an external modulator,
such as a Mach-Zehnder interferometer. Although such external
modulation schemes effectively encode the optical signals with
communication data, the external modulator is expensive and inserts
additional loss into the system. Such loss, however, can be
compensated in long haul networks with optical amplifiers, which
further add to the cost of the system.
Shorter haul networks, however, are more cost sensitive than long
haul networks. Accordingly, in order to reduce the cost of these
networks, semiconductor distributed feedback (DFB) directly
modulated lasers have been proposed. These lasers are turned on and
off directly in accordance with the communication data, thereby
eliminating the need for an external modulator. Further, since DFB
lasers can generate a high power optical output, few, if any,
optical amplifiers are required.
When the DFB laser is in the "on" state, however, a relatively
large current is injected into the semiconductor laser, while in
the "off" state a relative low current is injected and a small
amount of light is output. Such changes in current result in
corresponding changes in the carrier density within the laser,
which, in turn, alter output frequency and spectrally broaden or
"chirp" the emitted light.
As shown in FIG. 1, the optical spectrum of a directly modulated
semiconductor laser has a main intensity peak 101 at the intended
channel frequency. The optical spectrum, however, is spectrally
broadened to include a subsidiary peak 102 at chirp-induced
frequencies lower than the channel frequency.
The chirped signal includes relatively significant "blue" (higher
frequency at peak 101) and "red" (lower frequency at peak 102)
components, which can propagate through an optical fiber at
different speeds due to chromatic dispersion. Accordingly, light
from one pulse can overlap with a successive pulse at the receiving
end of an optical fiber causing increased bit error rate
probabilities. Directly modulated lasers therefore limit the
distance of short haul optical communication systems.
SUMMARY OF THE INVENTION
Consistent with the present invention, an optical device is
provided which comprises a semiconductor laser configured to be
coupled to a first end portion of an optical communication path,
and a drive circuit coupled to the semiconductor laser. The drive
circuit supplies an electrical signal to directly modulate the
semiconductor laser.
The optical device further includes an optical receiver configured
to be coupled to a second end portion of the optical communication
path to thereby receive the light emitted by said semiconductor
laser. In addition, an in-line fiber Bragg grating is provided in
the optical communication path. The in-line fiber Bragg grating is
spaced a first distance from the first end portion of the optical
communication path and a second distance from the second end
portion of the optical communication path, such that the first
distance is less than said second distance. Moreover, the in-line
fiber Bragg grating is provided in an in-transmission configuration
to substantially reject chirp-induced frequencies.
BRIEF DESCRIPTION OF THE DRAWINGS
Advantages of the present invention will be apparent from the
following detailed description of the presently preferred
embodiments thereof, which description should be considered in
conjunction with the accompanying drawings in which:
FIG. 1 illustrates an optical spectrum of a conventional directly
modulated DFB laser;
FIG. 2 illustrates a simplified schematic diagram of an optical
communication system in accordance with the present invention;
FIG. 3 illustrates a detailed schematic of a transmitter and
associated in-line fiber Bragg grating in accordance with an aspect
of the present invention;
FIG. 4 illustrates a transmissivity characteristic of an in-line
fiber Bragg grating and optical spectrum of a directly modulated
DFB laser in accordance with an aspect of the present
invention;
FIG. 5 illustrates an optical spectrum generated in accordance with
the present invention; and
FIG. 6 illustrates bit error rate plots achieved in accordance with
the present invention and obtained with corventional directly
modulated DFB lasers.
DETAILED DESCRIPTION
In accordance with the present invention, an in-line fiber Bragg
grating is coupled to the output of a directly modulated DFB laser.
The grating preferably rejects chirp induced frequencies of light
emitted by the DFB laser. Accordingly, light transmitted through
the grating is spectrally narrowed has a higher extinction ratio
and narrower spectrum. Accordingly, lower bit error rate
probabilities can be achieved.
Turning to the drawings in which like reference characters indicate
the same or similar elements in each of the several views, FIG. 2
illustrates a simplified schematic diagram of an optical
communication system in accordance with the present invention. The
optical communication system includes transmitter 210 coupled to an
optical communication path, such as an optical fiber 220, which
supplies optical communication signals to receiver 230. As
generally understood, receiver 230 can include photodiodes (not
shown) for sensing the optical signals and other appropriate
circuitry. As further shown in FIG. 2, optical fiber 220 includes
in-line fiber Bragg grating 240 provided in an "in transmission"
configuration. That is, light transmitted through the grating is
passed to receiver 230, while reflected light is rejected. In-line
fiber Bragg grating 240 is commercially available from Sumitomo
Electric Industries, Ltd. and 3M Specialty Optical Fibers, for
example.
It is understood that combiners, couplers, star distribution
networks, switching elements, optical amplifiers, signal
regenerators, reconditioners, add/drop multiplexers, and repeaters
and the like may be present in the optical communication system and
coupled to the optical communication path without any loss of
generality of applicability for the principles of the present
invention.
Transmitter 210 is shown in greater detail in FIG. 3. Transmitter
210 typically includes a drive circuit 315 that turns laser 325
"on" and "off" to transmit communication data as a series of
optical pulses on optical communication path 220. Laser 325 is
typically a DFB laser generally comprising III-V semiconductor
materials and commercially available from a wide variety of
suppliers such as Fujitsu, Alcatel, Lucent and Hewlett-Packard.
As seen in FIG. 4, output spectrum 410 of directly modulated laser
325 is superimposed on transmissivity characteristic 415 of in-line
fiber Bragg grating 240. Typically, transmissivity characteristic
415 has a low transmissivity over a first range of frequencies
substantially between .nu..sub.1 and .nu..sub.2, a high
transmissivity segment 419 over a second range of frequencies
greater than or equal to .nu..sub.3, and an intermediate portion
421 having a relatively steep slope typically at least 1 dB/GHz and
preferably 2-5 dB/GHz or more over a relatively narrow range of
frequencies between .nu..sub.2 and .nu..sub.3. The transmissivity
characteristic has an additional transmission maximum over
frequencies less than .nu..sub.4.
As further shown in FIG. 4, in-fiber Bragg grating 240, in the
in-transmission configuration, is preferably designed so that
chirp-induced frequencies, represented by the cross-hatched area
425 beneath optical spectrum curve 410, typically fall within
reduced-transmission portions of the transmissivity characteristic,
for example, between .nu..sub.1 and .nu..sub.3. Typically, the
difference in frequency between .nu..sub.1 and .nu..sub.3 is about
40 GHz. Moreover, this portion of the transmissivity characteristic
preferably has a loss greater than 3 dB, and is typically greater
than 10 dB.
The grating is further designed so that the peak optical power
frequency .nu..sub.P is greater than .nu..sub.3. As a result, the
chirp-induced frequencies of light emitted by laser 325 are
substantially rejected by grating 240, while the channel frequency,
i.e., .nu..sub.P, is transmitted. As shown in FIG. 5, the resulting
spectrum of light transmitted to receiver 230 has a single
intensity peak 510 at channel frequency .nu..sub.P, is narrowed
significantly, and is substantially free of the chirp-induced
frequencies. Typically, in order to insure that modulation induced
sidebands of the transmitted signal , not shown in FIG. 5, are not
rejected by grating 240, the grating is typically designed to have
a low loss (i.e., high transmissivity) over a range of frequencies
.nu..sub.p .+-..nu..sub.mod, where .nu..sub.mod is the modulation
frequency of laser 325. The loss is preferably less than 1 dB and
is typically less than 0.1 dB.
Moreover, the ratio of transmitted light in the on-state to
transmitted light in the off-state (the "extinction ratio") can be
achieved with the present invention with a smaller current swing
than that required by a conventional directly modulated DFB laser.
As noted above, current is continuously supplied to the laser in
both the on and off states, although the on-current is necessarily
significantly more than the off-current. In the conventional
directly modulated laser, the off-current must be diminished
substantially to a current slightly above the threshold current of
the laser in order to insure that the combined amounts of intended
channel light and the chirp light is sufficiently low. Consistent
with the present invention, however, the chirped light is rejected
by in-line fiber Bragg grating 240, and, therefore, a sufficiently
low light intensity can be achieved even at higher off-currents. As
a result, the off-current need not be diminished to the same extent
as in the conventional directly modulated scheme, and smaller
on-to-off current swings can be tolerated, thereby requiring a
simpler laser driver circuit. Conversely, the same swing of the
laser could yield a higher extinction ratio.
Improved bit error rates (BER) associated with the present
invention will now be discussed with reference to FIG. 6, which
illustrates log(BER) as a function of received optical power from a
directly modulated DFB laser. Curves 810 and 820 represent BER
characteristics of conventional directly modulated DFB lasers. In
particular, curve 810 corresponds to the BER of optical signals
received over 107 km of fiber having a dispersion of 17 ps/nm/km,
and curve 820 represents the BER of optical signals detected
adjacent the laser output (i.e., in, a "back-to-back"
configuration). Curves 830 and 840, however, correspond to BER
characteristics of optical signals transmitted in accordance with
the present invention: over 107 km of the fiber (curve 830) and
detected directly at the output of the grating in the back-to-back
configuration (curve 840) due to the higher extinction ratio.
As further seen in FIG. 6, for any given level of received optical
power, the BER associated with the present invention is
significantly lower than that of the conventional directly
modulated laser. Moreover, the BER of optical signals transmitted
in accordance with the present invention over 107 km of fiber,
which may have been expected to be relatively high, is actually
reduced considerably, and is even lower than the BER of a
conventional directly modulated laser measured in the back-to-back
configuration.
Returning to FIG. 3, by providing in-line fiber Bragg grating 240
closer to transmitter 210 than receiver 230, for example within 3.0
meters from the output of laser 325, a feedback control circuit 260
can be provided in transmitter 210 for regulating the frequency of
light output from laser 325 in accordance with the intermediate
portion 421 or "edge" of the transmissivity characteristic of
in-line fiber Bragg grating 240. Preferably, the difference between
.nu..sub.1 and .nu..sub.2 should be relatively large, typically at
least 40 GHz, in order to insure that the frequency is adjusted to
the appropriate edge of in-line fiber Bragg grating 240
Feedback control circuit 260 can include photodetectors (not shown)
for sensing light reflected by grating 240 via coupler 262 and
light transmitted through grating 240 via coupler 264. Electrical
signals generated by these photodetectors can then be used to
adjust the frequency of laser 325, for example, in a manner similar
to that described in the above cited patent application. In
addition, a back facet monitoring photodiode (not shown) may be
provided adjacent a read facet of laser 325 to monitor optical
power output.
It is further noted that light can be reflected by grating 240 back
to laser 325, thereby degrading performance of the laser.
Accordingly, as further shown in FIG. 3, an optical isolator 335 is
typically provided between the output of laser 325 and in-line
fiber Bragg grating 240 in order to prevent any reflected light
from reaching laser 325.
While the foregoing invention has been described in terms of the
embodiments discussed above, numerous variations are possible.
Accordingly, modifications and changes such as those suggested
above, but not limited thereto, are considered to be within the
scope of the following claims.
* * * * *